1. Field of the Invention
This invention relates to precipitation-hardenable aluminum alloys, and, particularly, to nitrided aluminum alloys having a high tensile-strength and high hardness as well as to the mother alloys therefor, and to the process for manufacturing these alloys.
More specifically, this invention concerns a process for manufacturing precipitation-hardenable nitrided aluminum alloys in which process the heretofore known nitrided aluminum alloys, which have needed a temperature as high as 900.degree. C and more for their manufacturing, are produced according to the invention through the addition of nitrided mother alloys to render possible the use of ordinary aluminum alloy melting furnaces resistant only up to about 800.degree. C and to obtain precipitation-hardenable nitrided aluminum alloys having further improved properties and composition-uniformity.
2. Description of the Prior Art
Although the invention is useful for significantly improving properties of not only the nitrided alloys based upon aluminum but also the nitrided alloys based upon other metals than aluminum, the invention is particularly useful when used in the case of the nitrided alloys based upon aluminum. Therefore, the invention is described hereinbelow essentially with respect to the aluminum alloys.
Alluminum has the advantages such as low specific weight, high anticorrosion property, high anodizing property and high formability, but this metal also has some disadvantages when used as structural materials such as, for example, low tensile strength and hardness and poor abrasion resistance. For the purpose of utilizing the advantages of aluminum as described above and eliminating the disadvantages thereof as mentioned above, there have been developed a series of precipitation-hardenable alloys called "Duralumin" or "17S" in the USA such as, for example, Al -- Cu -- Mg alloy (JIS*No. 2000 and thereafter), Al -- Mg -- Si alloys (JIS No. 6000 and thereafter), or Al -- Zn -- Mg -- Cu alloys (JIS No. 7000 and thereafter) called "Extra Super Duralumin" or "75S" in the USA. In all of these alloys, the quality can be improved by only adding suitable component elements to the aluminum metal, and the tensile strength as well as the hardness, which is substantially comparable with those of low-carbon steel materials, can be obtained by subsequent heat treatment or solution heat treatment. In fact, a Duralumin such as JIS 7075 - T.sub.6 has a tensile strength near to that of JIS S 30C structural carbon steel. FNT * JIS: Japanese Industrial Standards.
Physical properties of some commercial aluminum alloys under several "Industrial Standards" are shown in Table 1. For the comparison purpose, the data on the alloys obtained in the following Examples 1 and 2 in accordance with the present invention are also shown in Table 1.
TABLE 1 ______________________________________ Tensile Yield Elon- Strength Strength ga- Hardness kg/mm.sup.2 kg/mm.sup.2 tion % HmV ______________________________________ JIS* 7075 T.sub.6 ** 57 51 7 Ca.180 (Aluminum alloy) JIS S 30C (Carbon 55 34 23 -- steel) JIS S 40C (Carbon 58 40 22 -- steel) ISO* AlZn6MgCu 55.5 50.0 7 -- AA* 7075 - T.sub.6 ** 58.5 51.5 11 -- DIN* AlZnMgCu0.5 50 43 7 -- AlZnMgCu1.5 53 47 7 -- CSA* ZG 62 - T.sub.6 57 51.5 7 -- Example 1 70 -- 10 230 Example 2 65.5 -- 10.7 230 ______________________________________ Note *JIS: Japan ISO: International AA: The United States of America DLN: Germany CSA: Canada **The T.sub.6 -treatment comprises solution heating and precipitation treatment for maximum strength as shown in American Aluminum Association Aluminum Standards and Data 1972 - 1973, page 9, and particularly the solution heating was at ca. 450.degree. C and the precipitation treatment was at ca. 120.degree. C.
The compositions in % of these aluminum alloys listed in Table 1 are shown in Table 1 bis.
TABLE 1 bis __________________________________________________________________________ Present* Patent** JIS ISO AA DIN CSA Invention 728,024 7075 AlZn6MgCu 7075 AlZnMgCu1.5 AlZnMgCu0.5 ZG62 __________________________________________________________________________ Zn 3.2 - 8.0 3.2 - 8.0 5.1 - 6.1 5.1 - 6.4 5.1 - 6.1 5.1 - 6.1 4.3 - 5.2 5.1 - 6.1 Mg 1.2 - 4.5 1.2 - 4.5 2.1 - 2.9 2.1 - 2.9 2.1 - 2.9 2.1 - 2.9 2.6 - 3.6 2.1 - 2.9 Cu 0.1 - 1.0 0.3 - 1.5 1.2 - 2.0 1.2 - 2.0 1.2 - 2.0 1.2 - 2.0 0.5 - 1.0 1.2 - 2.0 Cr 0.1 - 0.5 0.1 - 0.5 0.18 - 0.35 0.1 - 0.35 0.18 - 0.35 0.18 - 0.3 0.1 - 0.3 0.18 - 0.35 Zr or Zr Ti Ti + Zn Ti 0.01 - 1.2 0.05 - 0.22 0.3 0.2 0.2 0.2 0.2 Fe -- 0.2 - 1.2 0.5 0.5 0.5 0.5 0.5 0.5 Si -- 0.05 - 0.85 0.4 0.4 0.4 0.5 0.5 0.4 Ni or Ni Co 0.2 - 1.2 0.2 - 1.2 -- 0.1 -- -- -- -- Mn + Cr Mn -- 0.1 - 1.2 0.3 0.5 0.3 - 0.3 0 - 0.4 0.3 Be 0.02 - 1.0 0.02 - 1.0 -- B 0.005 - 0.2 0.005 - 0.2 -- __________________________________________________________________________ *The data from Table 5 to follow. **Japanese Patent No. 728,024 which is referred to hereinbelow.
However, along with the development of industry, an aluminum alloy having a higher strength and a lower specific gravity is desired, and various means for satisfying this desire have been attempted. For example, there has been recently developed a process for producing high-tensile strength, high-hardness aluminum alloys by using a high-temperature nitriding process, such as described in Japanese Patent No. 621,486 and No. 728,028. (Japanese Patent Publication Nos. 11411/1971 and 31807/1973, respectively.)
The high-tensile strength, high-hardness aluminum alloy as mentioned above (referred to hereinbelow as "the prior nitrided aluminum alloys") has a tensile strength of over 70 kg/mm.sup.2 and a Vickers hardness over 220, but it has disadvantages such as low ductility (referred to hereinbelow as "extrusion property") and low anodizing property.
Further, since the nitriding process in the manufacture of "the prior nitride aluminum alloys" is carried out by blowing nitrogen or ammonia into the melt, which is in a total quantity to be nitrided, at a temperature as high as 900.degree. C - 1300.degree. C, this temperature range exceeds the normal heat resistance of 800.degree. C for standard aluminum alloy melting furnaces. Consequently, the process of the "prior nitrided aluminum alloys" can not be practiced in the usual metallurgical works. Nitriding treatment of a large quantity of alloy in a special large-sized and expensive high-temperature furnace requires very careful process control and furnace maintenance, large fuel consumption, and, thus, increased manufacturing costs for the alloy produced.